This invention relates in general to power supplies and in particular to a damping system for EMI filters.
All electromechanical equipment create electromagnetic interference or "noise" on their power lines. The noise can be generated by load transients, switching power supplies or PWM inverters. Such noise can adversely affect the performance of other equipment that receive power from same power lines.
Typically, the noise is removed from the power lines by an EMI filter. As shown in FIG. 1, the EMI filter 2 includes an inductor L and capacitor C coupled between an input voltage bus 4a, 4b and a DC link 6a, 6b. This filter has a resonant frequency of 1/(2.pi..sqroot.LC). The EMI filter 2 passes all signals having frequencies above the resonant frequency and attenuates all signals having frequencies below the resonant frequency. However, it amplifies signals having frequencies at or near the resonant frequency. As a result, the peak voltage at the resonant frequency exceeds the voltage on the bus 4a, 4b. For an undamped EMI filter 2 having an inductance of 1 mH and a capacitance of 22 microfarads, the peak at the resonant frequency of 1 kHz is 18dB, which is 180% of the DC link voltage (see FIG. 2). Consequently, the ratings of the components, including the filter capacitor C, on the DC link 6a,6b must be increased above the maximum peak voltage.
The EMI filter 2 can be equipped with a damping circuit which limits peak voltage and ringing caused by load and line transients. Typically, this damping circuit includes a resistor in series with a capacitor. The damping resistor dissipates the energy stored in the filter 2 and the damping capacitor blocks the DC component of the input voltage, thereby reducing dissipation by the damping resistor. The size of the damping capacitor depends upon the maximum tolerable peak voltage. A maximum overshoot of 5-6 dB requires the damping capacitor to have a capacitance of approximately three times the filter capacitance. Further reduction in peaking requires an even larger damping capacitance, typically provided by a multilayer ceramic (MLC) capacitor.
The damping capacitor tends to be heavy and expensive. Further, MLC capacitors are only offered in discrete voltages (e.g., 100 vdc, 200 vdc, 400 vdc). Consequently, a power line voltage of 110 vdc requires an MLC capacitor of 200 vdc. Still further, link voltage peaking caused by moderate damping can force the use of bigger capacitors connected to the DC link.
The cost, size and weight of the capacitance increases with the power rating of the power supply system.